Using gas flow to clean and transfer materials is a flexible and convenient process in many industrial applications. However, an associated problem is how to effectively separate and remove fine solid particles (e.g., dust or sand) from the gas stream before exhausting the gas to the atmosphere, or before the gas reaches the power source (e.g., a fan or blower).
For mobile equipment, such as street sweepers, sewage trucks, and industrial vacuum trucks, this problem is especially challenging. Due to the limited availability of power and space, any dust separation device on mobile equipment should be highly efficient. In general, it should employ a low pressure drop, or decrease in suction power. Further considerations tend toward a compact and lightweight system, taking up little space with a reduced payload. Any dust separation device or system on mobile equipment should be reliable and have little down-time.
A common cyclone may cause a 12 to 14 inch water column pressure drop at a flow velocity of 70 feet per second. Such a device would not be very suitable for use on a street sweeper, where the maximum suction power is generally a 48 inch water column.
Many devices for removing materials entrained in a gas flow currently exist. Using filters can usually achieve high separation quality, but filters are also generally associated with high pressure loss, high cost, and maintenance. Filters may also have problems when dealing with high temperature gases, depending on the material of the filter. In comparison, mechanical separation devices such as inertial inlet separators and cyclones have many advantages. They are simple, low-cost, and generally maintenance-free. But while an inertial inlet separator is the simplest, it is generally limited by low separation efficiency and larger space demands to settle and collect dust.
Cyclones are another option. Most widely-used cyclones are reverse flow type, as shown in FIG. 1 of this disclosure. The dust-laden gas stream is introduced tangentially into a cylindrical barrel or chamber to generate rotational flow moving downward. Larger or heavier particles are forced to move outwardly to the walls and then fall into the dust collector at the bottom. In this version, the cleaned gas stream will then reverse direction at 180-degrees to exit from a tube at the top end. This type of cyclone is well-known in the art for its high efficiency, typically on the order of 90% or greater, depending on the application. However, one drawback of all traditional cyclones is that the high efficiency comes at the price of high pressure drop, especially in high flow velocity situations. To overcome this problem, one solution is to use multiple cyclones in parallel arrangement to reduce the flow to each cyclone so that the pressure loss is reduced. For this reason, it is not surprising to see an industrial vacuum truck having a row of 4 to 6 cyclones attached around the debris body to handle a flow in the range of 3000 to 5000 cubic feet per minute. But such an alternative leads to other problems, as it requires more space, parts, and often has difficulty evenly distributing flow between the cyclones.
Alternatively, co-current flow type cyclones, as shown in FIG. 2 of this disclosure, lead to less pressure loss but generally need a scavenging flow to most effectively remove the dust from the separator. Without a scavenging flow, the separation may decrease by 20% or even more. For applications such as engine inlet gas cleaning, it is common to use the engine exhaust gas to provide the needed scavenging flow. In a stationary application, such as gas cleaning system in a production plant, a powered suction passage with filters is sometimes used in a co-current cyclone. Further, co-current cyclones using guide vanes to generate spiral flow are generally more vulnerable to wear and tear by sand, or clogging due to paper, leaves, or rope in street sweeper applications.
When dust separators are used in a mobile vacuum equipment application, such as a street sweeper, the separated dust is typically maintained or stored within the equipment, instead of being exhausted into the environment. Using an extra suction source with filtration capability to collect dust, such as is often required in a co-current flow cyclone, is less attractive from both a power and space stand point.